Ear-body lift and a novel thrust generating mechanism revealed by the complex wake of brown long-eared bats (Plecotus auritus)

Abstract

Large ears enhance perception of echolocation and prey generated sounds in bats. However, external ears likely impair aerodynamic performance of bats compared to birds. But large ears may generate lift on their own, mitigating the negative effects. We studied flying brown long-eared bats, using high resolution, time resolved particle image velocimetry, to determine the aerodynamics of flying with large ears. We show that the ears and body generate lift at medium to cruising speeds (3–5 m/s), but at the cost of an interaction with the wing root vortices, likely reducing inner wing performance. We also propose that the bats use a novel wing pitch mechanism at the end of the upstroke generating thrust at low speeds, which should provide effective pitch and yaw control. In addition, the wing tip vortices show a distinct spiraling pattern. The tip vortex of the previous wingbeat remains into the next wingbeat and rotates together with a newly formed tip vortex. Several smaller vortices, related to changes in circulation around the wing also spiral the tip vortex. Our results thus show a new level of complexity in bat wakes and suggest large eared bats are less aerodynamically limited than previous wake studies have suggested.

Mechanical power curve measured in the wake of pied flycatchers indicates modulation of parasite power across flight speeds

“…The reasons for this have been speculated on, with a focus on what makes the bats less efficient than birds [8][9][10]. One likely contributing factor is the protruding ears of bats that seem to affect the flow over the body to increase drag and reduce the generation of lift by the body compared with birds [10][11][12]. However, so far, no one has asked what it is that makes birds more efficient than bats.…”

Flow pattern similarities in the near wake of three bird species suggest a common role for unsteady aerodynamic effects in lift generation

“…At low Reynolds numbers, the flow may develop wake-wake interactions that can potentially cause cancellation of vorticity and reconnection of vortex lines. Recently, Johansson et al [27] argued that the unsteady flow has an impact on the flight cost. These findings can significantly modify the qualitative and quantitative properties of fluid flows [6].…”

Inspiration for wing design: how forelimb specialization enables active flight in modern vertebrates

“…Active upstrokes, on the other hand, enhance manoeuvrability by reducing pitch stability [125,153]. Bats compensate for this reduced stability and enhanced pitch and yaw control by pitching their wings at the end of their upstroke [133].…”

“…At low speeds, the rotation of the hand-wing manifests as a backward flick generating thrust and weight support [118,131,132], but at high speeds, the upstroke generates only weight support [84,132]. Bats may also generate thrust and negative lift at the end of their upstroke as the wing moves up with a negative angle of attack [115,126,[132][133][134].…”

“…Hummingbirds are aided by their small mass, high wingbeat frequency and active upstrokes for performing these manoeuvres [104,143]. Bats are also thought to use active upstrokes [114,131,133,144], and further benefit from a combination of lower wing loading [114] and ability to actuate the numerous joints in their wings to adapt their shape [87,100,145]. Although lower mineralization reduces the weight of their wing bones [24], the combination of solid bones (figure 2c) and muscular membranes (figure 5a) makes bat wings proportionally heavier than those of all other extant flyers [145].…”